1. Field of the Invention
The present invention relates to a film formation method and a film formation device for forming, on a substrate, a film containing constituent elements of a target through a vapor deposition process using plasma. The present invention also relates to a piezoelectric film formed with the film formation method, as well as a piezoelectric device and a liquid discharge device including the piezoelectric film.
2. Description of the Related Art
Sputtering is a film formation method, in which a substrate and a target are placed to face to each other, and a gas which is plasmized under reduced pressure is made to collide against the target, so that the energy of the collision makes molecules or atoms be ejected from the target and deposited on the substrate.
Theoretically, the composition of the film formed through sputtering should be substantially the same as the composition of the target. However, if the constituent elements of the film include an element with high vapor pressure, the element tends to be reverse sputtered out of the surface of the formed film, and this may often make it difficult to provide the film having substantially the same composition as the composition of the target.
The reverse sputtering is a phenomenon in which, if there is a large difference in ease of sputtering (sputter rate) among the constituent elements, a constituent element which is more susceptible to sputtering than the other constituent elements among the constituent elements deposited on the substrate is preferentially sputtered out of the surface of the film, although the constituent elements sputtered from the target have almost the same composition as the composition of the target.
For example, in PZT (lead zirconate titanate), which is a perovskite oxide with high ferroelectricity, or in a substitution system thereof, Pb is more susceptible to the reverse sputtering than Ti and Zr, and thus the Pb concentration in the film tends to be lower than the Pb concentration in the target. Also, in a perovskite oxide containing Bi or Ba at the A-site, these elements have high vapor pressure and thus have the similar tendency.
As another example, Zn element in Zn-containing compounds has high vapor pressure and thus has the similar tendency. For example, in a zinc oxide transparent conductor film or transparent semiconductor film, such as InGaZnO4 (IGZO), which has excellent electric and optical characteristics comparable to indium tin oxide (ITO) and which is inexpensive and is an abundant resource, Zn is more susceptible to the reverse sputtering than the other constituent elements, and the composition of the film tends to contain less Zn than that in the composition of the target.
For the systems as presented in the above examples, in order to obtain a desired composition, such countermeasures have been taken as using a target having a composition in which content of the element susceptible to the reverse sputtering is higher than the desired composition. However, in order to stably provide a desired composition, it is preferred to be able to form a film having substantially the same composition as the composition of the target.
The above-described problem is not limited to the case of sputtering. Similar problem may occur in other film formation methods in which a substrate and a target are placed to face to each other and a film containing constituent elements of the target is formed on the substrate through a vapor deposition process using plasma.
In order to minimize the composition shift as described above, the present inventors have proposed in U.S. Patent Application Publication Nos. 20080081216 and 20080081128 to control a temperature during film formation Ts (° C.) and a potential difference Vs−Vf (V) between a plasma potential Vs (V) in plasma and a floating potential Vf (V).